Ammonia oxidation/decomposition catalyst

ABSTRACT

Provided is an ammonia oxidation/decomposition catalyst which can decrease the reduction temperature of a support, which is required for the catalyst to have a property of being activated at room temperature, and also can render a property of being activated at a temperature lower than room temperature. The ammonia oxidation/decomposition catalyst of the present invention is an ammonia oxidation/decomposition catalyst, comprising: a catalyst support composed of a composite oxide of cerium oxide and zirconium oxide; and at least one metal selected from the group consisting of metals of group 6A, group 7A, group 8, and group 1B as a catalytically active metal deposited thereon, characterized in that the molar concentration of zirconium oxide in the catalyst support is from 10 to 90%.

TECHNICAL FIELD

The present invention relates to an ammonia oxidation/decompositioncatalyst which is used in a combustion improver for an ammonia engineusing ammonia as a fuel or in a hydrogen production reaction in a fuelcell, etc.

BACKGROUND ART

Conventionally, in order to produce hydrogen by decomposing ammonia, itis necessary to allow a reaction of the following formula (I) to proceedin the presence of a ruthenium-based ammonia decomposition catalyst.

NH₃

3/2H₂+1/2N₂   (I)

ΔH_(298K)=46.1 kJ/mol

Since the reaction of the formula (I) is an endothermic reaction, inorder to obtain a stable ammonia decomposition ratio, it is necessary toprovide heat to a reaction system and set the reaction temperature to350° C. or higher.

Therefore, in order to suppress a decrease in gas temperature due to theendothermic reaction, heat is supplied from the outside conventionally.However, by this method, the heat transfer rate is lower than thereaction rate, and therefore, in order to obtain a sufficient heattransfer rate, there is no other choice but to increase a heat transferarea and it is difficult to decrease the size of an apparatus.

A method in which exhaust gas of an engine or the like is used as a heatsource for supplying heat from the outside is also contemplated,however, this method has a drawback that, in the case where thetemperature of the heat source is 350° C. or lower, since thetemperature of the heat source is lower than the temperature at which acatalyst works, and therefore, heat cannot be supplied and apredetermined amount of hydrogen cannot be produced.

As the heat source for supplying heat, other than the supply from theoutside, there is a method in which as shown in the following formula(II), heat is generated by a catalytic reaction between ammonia andoxygen, and the generated heat is used.

NH₃+3/4O₂

1/2N₂+3/2H₂O   (II)

ΔH_(298K)=−315.1 kJ/mol

When the reaction of the formula (I) and the reaction of the formula(II) are caused in the same reaction tube, the heat absorbed by thereaction of the formula (I) can be supplemented with the heat generatedby the reaction of the formula (II). Further, the temperature of acatalyst layer can be controlled by controlling the amount of oxygen inthe formula (II). For example, in the case where the temperature of thesupply gas preheated by waste heat of engine exhaust gas through heatexchange varies, hydrogen can be stably produced.

As an ammonia oxidation catalyst to be used for allowing the reaction ofthe formula (II) to proceed, a platinum-based catalyst is generallyused. For example, in Patent Literature 1, a multilayer ammoniaoxidation catalyst containing a refractory metal oxide, a platinum layerprovided on this refractory metal oxide, and a vanadia layer provided onthe platinum is proposed.

However, the operating temperature of the catalyst is about 200° C., andthe oxidation reaction cannot be allowed to proceed at a temperature ofabout 200° C. or lower, and therefore, it is necessary to increase thegas temperature to about 200° C. with an electric heater or the like.

In Patent Literature 2, an ammonia oxidation catalyst containing anoxide of at least one element selected from cerium and praseodymium, anoxide of at least one element selected from non-variable valency rareearth elements including yttrium, and cobalt oxide is proposed, and inPatent Literature 3, an ammonia oxidation catalyst, which containsfilaments composed essentially of platinum, rhodium, and optionallypalladium, and in which the filaments have a platinum coating isproposed. However, these catalysts also have the same problem as inPatent Literature 1.

PTL 1: JP-T-2007-504945, the term “JP-T” as used herein means apublished Japanese translation of a PCT patent application

PTL 2: Japanese Patent No. 4,165,661

PTL 3: JP-A-63-72344

SUMMARY OF INVENTION Technical Problem

The present inventors of this application developed an ammoniaoxidation/decomposition catalyst obtained by depositing a catalyticallyactive metal on a support composed of a metal oxide which can undergo aredox reaction.

According to this ammonia oxidation/decomposition catalyst, (i) bybringing ammonia and air into contact with this catalyst at roomtemperature, first, the support in a reduced state is reacted withoxygen to generate oxidation heat, and the temperature of a catalystlayer is instantaneously increased and reaches a temperature at whichammonia and oxygen are reacted with each other; and (ii) thereafter, anammonia oxidation reaction which is an exothermic reaction proceedsspontaneously, and heat generated by the exothermic reaction is used inthe process of decomposing ammonia in the presence of the catalyticallyactive metal according to the above formula (I), whereby hydrogen isproduced.

Therefore, by using the above catalyst, preheating with an electricheater or the like is no longer required, and therefore, the hydrogenproduction cost can be reduced.

By reducing the support in advance, the support in a reduced state isreacted with oxygen to generate oxidation heat when oxygen comes intocontact with the support at room temperature, whereby the above catalysthas a property of being activated at room temperature.

In the case of using CeO₂ alone as a support, in order to render aproperty of being activated at room temperature, it was necessary toreduce the support at a high temperature of 600° C. or higher.

Moreover, although the above catalyst has a property of being activatedat room temperature, a property of being activated at a temperaturelower than room temperature could not be exhibited. If the catalyst canbe activated at a temperature even lower than room temperature, therange of application of the catalyst can be widened, and therefore sucha catalyst is convenient.

The invention has been made in view of the above circumstances, and anobject of the invention is to provide an ammonia oxidation/decompositioncatalyst which can decrease the reduction temperature of a support,which is required for the catalyst to have a property of beingactivated, and also can render a property of being activated at atemperature lower than room temperature.

Solution to Problem

In order to achieve the above object, the ammoniaoxidation/decomposition catalyst of the invention is an ammoniaoxidation/decomposition catalyst, containing: a catalyst supportcomposed of a composite oxide of cerium oxide and zirconium oxide; andat least one metal selected from the group consisting of metals of group6A, group 7A, group 8, and group 1B as a catalytically active metaldeposited thereon, and is characterized in that the molar concentrationof zirconium oxide in the catalyst support is from 10 to 90%.

Preferably, the ammonia oxidation/decomposition catalyst is in ahoneycomb form.

Preferably, the ammonia oxidation/decomposition catalyst is in a pelletor raschig ring form.

The catalyst support which can undergo a redox reaction and is used inthe ammonia oxidation/decomposition catalyst of the present invention iscomposed of a composite oxide of cerium oxide and zirconium oxide, andthe molar concentration of zirconium oxide in the catalyst support isfrom 10 to 90%, more preferably from 20 to 70%.

The catalytically active metal to be deposited on the catalyst supportis preferably at least one metal selected from the group consisting ofmetals of group 6A such as Mo and Cr, metals of group 7A such as Mn,metals of group 8 such as Ru, Pt, Rh, Pd, Co, Ni, and Fe, and metals ofgroup 1B such as Cu and Ag.

In the ammonia oxidation/decomposition catalyst according to the presentinvention, a part of or the whole of the metal oxide constituting thecatalyst support is reduced by the following reaction by performing aheating treatment at 200 to 400° C. in a hydrogen stream or in anammonia stream.

CeO₂+xH₂→CeO_(2-x)+xH₂O (0<x<2)

CeO₂+2x/3NH₃→CeO_(2-x)+xH₂O+x/3N₂ (0<x<2)

The catalyst after undergoing the above reduction is used in an ammoniaoxidation/decomposition reaction. The reduction treatment of the ammoniaoxidation/decomposition catalyst may be performed either before or afterloading the catalyst in a catalyst reactor.

When the ammonia oxidation/decomposition catalyst in a state in whichthe catalyst support is reduced is brought into contact with ammonia andair at room temperature or a temperature between −30 and −15° C., whichis equal to or lower than room temperature, first, the catalyst supportin a reduced state is reacted with oxygen to generate oxidation heat,and the temperature of the catalyst layer is instantaneously increased.Once the temperature of the catalyst layer is increased to a temperature(200° C.) at which ammonia and oxygen are reacted with each other,thereafter an ammonia oxidation reaction proceeds spontaneouslyaccording to the above formula (II). Heat generated by the exothermicreaction of the formula (II) is used in the process of decomposingammonia in the presence of the catalytically active metal according tothe above formula (I), whereby hydrogen is produced.

Advantageous Effects of Invention

According to the present invention, in an ammoniaoxidation/decomposition catalyst obtained by depositing at least onemetal selected from the group consisting of metals of group 6A, group7A, group 8, and group 1B as a catalytically active metal, by using acatalyst support composed of a composite oxide of cerium oxide andzirconium oxide, and setting the molar concentration of zirconium oxidein the catalyst support to 10 to 90%, a decrease in the reductiontemperature of the support, which is required for the catalyst to have aproperty of being activated, can be achieved, and also a property ofbeing activated at a temperature lower than room temperature can berendered.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing a change over time of the temperature of acatalyst layer when a test for a property of being activated at a lowtemperature was performed.

FIG. 2 is a graph showing a change over time of the concentration ofeach gas when a catalyst of Example 17 was used.

DESCRIPTION OF EMBODIMENTS

Hereinafter, several Examples of the present invention and ComparativeExamples for comparison therewith will be described for specificallyillustrating the present invention.

a) Catalyst Support

As the catalyst support, four types of commercially available CeO₂—ZrO₂(manufactured by DAIICHI KIGENSO KAGAKU KOGYO Co., LTD.) in which themolar concentration of ZrO₂ is 10, 20, 50, and 80% were used.

b) Deposition of Catalytically Active Metal

A catalytically active metal was deposited on each of the above catalystsupports. As the catalytically active metal, Ru, Pt, Rh, and Pt—Rh,which are noble metals, and Co, Ni, Fe, Cu, Mo, and Mn, which are basemetals, were used. The catalyst deposition amount was set to 2% byweight in all cases.

(Preparation of Pellet Catalyst)

The deposition of each of the above metals on the catalyst support wasperformed as follows. Each of the metal salts, which are precursors ofthe respective metals, was dissolved in pure water, to which the abovecatalyst support was immersed so as to make the metals dispersed on thesupports such that the deposition amount of the catalytically activemetal was 2% by weight (in terms of metal).

This dispersion was heated to gradually evaporate water (an evaporationto dryness method).

The obtained powdery substance was calcinated in air at 300° C. for 3hours.

The powdery substance after firing was compression molded and theresulting molded product was sieved to 1 to 0.85 mm and used.

(Preparation of Honeycomb Catalyst)

The catalytically active metal was deposited on 600 cpi cordierite by awash-coating method until the catalyst deposition amount reached about250 g/L.

As the catalyst support for Comparative Examples, commercially availableCeO₂ (manufactured by DAIICHI KIGENSO KAGAKU KOGYO Co., LTD.) was used.On this catalyst support, each of the catalytically active metals, Ru,Co, and Ni, were deposited. The catalyst deposition amount was set to 2%by weight.

c) Test for Property of Being Activated at Room Temperature (ReductionTreatment of Catalyst)

After each of the obtained pellet catalysts (1 g) and the honeycombcatalysts (4 mL) was loaded in a flow-through type reaction tube,several reduction treatments were performed at different temperatures toeach treatment in a hydrogen stream. Each of temperatures of thereduction treatment was a range from 150° C. to 800° C. and each had 50°C. intervals. The reduction times for each treatment were set to 2hours.

(Validation of Property of Being Activated at Room Temperature)

The above catalyst loaded in the reaction tube and undergoing thereduction treatment at each temperature was kept at 25° C. in a nitrogenatmosphere, and thereafter, oxygen (air) and ammonia were simultaneouslysupplied to the catalyst layer. The ammonia supplying amount was keptconstant at 2.5 NL/min, and the air supplying amount was set such thatthe volume ratio of air to ammonia was 1.0. The temperature of thecatalyst layer and the gas composition at the outlet were measured by athermocouple and a mass spectrometer, respectively.

From the above results, a catalyst that satisfied the following threerequirements: the temperature of the catalyst layer was increased; theproduction of hydrogen was observed; and hydrogen was stably producedfor 30 minutes or more was determined as a catalyst that exhibits aproperty of being activated at room temperature, and with respect tosuch a catalyst, the temperature required for the reduction treatmentwas determined as the reduction temperature.

The form and the composition of each of the catalysts of Examples andComparative Examples and the reduction temperature thereof are shown inthe following Table 1.

TABLE 1 Reduction Form Composition temperature Example 1 PelletRu/CeO₂—ZrO₂ (10 mol %) 400° C. 2 Pellet Ru/CeO₂—ZrO₂ (20 mol %) 250° C.3 Pellet Ru/CeO₂—ZrO₂ (50 mol %) 200° C. 4 Pellet Ru/CeO₂—ZrO₂ (80 mol%) 200° C. 5 Pellet Pt/CeO₂—ZrO₂ (50 mol %) 200° C. 6 PelletRh/CeO₂—ZrO₂ (50 mol %) 200° C. 7 Pellet Pt—Rh/CeO₂—ZrO₂ (50 mol %) 200°C. 8 Pellet Co/CeO₂—ZrO₂ (50 mol %) 300° C. 9 Pellet Fe/CeO₂—ZrO₂ (50mol %) 300° C. 10 Pellet Ni/CeO₂—ZrO₂ (50 mol %) 300° C. 11 PelletCu/CeO₂—ZrO₂ (50 mol %) 350° C. 12 Pellet Mo/CeO₂—ZrO₂ (50 mol %) 400°C. 13 Pellet Mn/CeO₂—ZrO₂ (50 mol %) 400° C. 14 Honey- Ru/CeO₂—ZrO₂ (50mol %) 200° C. comb Comparative Example 1 Pellet Ru (2 wt %)/CeO₂ 600°C. 2 Pellet Co (2 wt %)/CeO₂ 600° C. 3 Pellet Ni (2 wt %)/CeO₂ 700° C.

As apparent from the above Table 1, in the case where the catalystsupport was CeO₂ (Comparative Examples 1 to 3), in order to exhibit aproperty of being activated at room temperature, it was necessary to setthe reduction temperature to 600° C. or higher, while, in the case whereZr was added at 10 mol % to CeO₂ (Example 1), the reduction temperaturefor exhibiting a property of being activated at room temperature was400° C., and therefore, the reduction temperature could be decreased by200° C. as compared with Comparative Example 1.

It was confirmed that in the case where ammonia oxidation/decompositionproceeded and the temperature of the catalyst layer was increased to600° C. or higher, when the reaction was stopped, merely by blowingammonia into the catalyst layer, the temperature of the catalyst layerwas increased by the progress of the oxidation reaction (which is anexothermic reaction) with the supported catalyst, and a property ofbeing activated at room temperature was exhibited again. Accordingly,reactivation could be achieved under a milder condition.

From the above results, the molar concentration of Zr added wasdesirably from 20 to 70 mol %, and in the case where Ru, which is anoble metal, was used as the catalytically active metal, a property ofbeing activated at room temperature could be exhibited when thereduction temperature was 200° C., and in the case where Ni or Co, whichis a base metal, was used as the catalytically active metal, a propertyof being activated at room temperature could be exhibited when thereduction temperature was 300° C.

d) Test for Property of Being Activated at Low Temperature (ReductionTreatment of Catalyst)

After each of the obtained pellet catalysts (1 g) and the honeycombcatalysts (4 mL) was loaded in a flow-through type reaction tube, areduction treatment was performed in a hydrogen stream at 600° C. for 2hours.

(validation of Property of Being Activated at Low Temperature)

The above catalyst loaded in the reaction tube and undergoing thereduction treatment was cooled in a nitrogen atmosphere, and after thetemperature was reached a predetermined temperature, oxygen (air) andammonia were simultaneously supplied thereto. The ammonia supplyingamount was kept constant at 1 NL/min, and the air supplying amount wasset such that the volume ratio of air to ammonia was 1.0. Thetemperature of the catalyst layer and the hydrogen generation amount atthe outlet of the catalyst layer were measured by a thermocouple and amass spectrometer, respectively.

The form and the composition of each of the catalysts of Examples andComparative Example are shown in the following Table 2.

TABLE 2 Initial Ammonia temperature of decomposition Form Compositioncatalyst layer ratio (%) Example 15 Pellet Ru/CeO₂—ZrO₂ (10 mol %) −15°C. 41 16 Pellet Ru/CeO₂—ZrO₂ (20 mol %) −15° C. 40 17 PelletRu/CeO₂—ZrO₂ (50 mol %) −15° C. 36 18 Pellet Ru/CeO₂—ZrO₂ (70 mol %)−15° C. 33 19 Pellet Ru/CeO₂—ZrO₂ (90 mol %) −15° C. 35 20 PelletRh/CeO₂—ZrO₂ (50 mol %) −15° C. 37 21 Pellet Pt/CeO₂—ZrO₂ (50 mol %)−15° C. 39 22 Pellet Pt—Rh/CeO₂—ZrO₂ (50 mol %) −15° C. 42 23 PelletCo/CeO₂—ZrO₂ (50 mol %) −15° C. 33 24 Pellet Fe/CeO₂—ZrO₂ (50 mol %)−15° C. 32 25 Pellet Ni/CeO₂—ZrO₂ (50 mol %) −15° C. 35 26 PelletCu/CeO₂—ZrO₂ (50 mol %) −15° C. 20 27 Pellet Mo/CeO₂—ZrO₂ (50 mol %)−15° C. 28 28 Pellet Mn/CeO₂—ZrO₂ (50 mol %) −15° C. 29 29 PelletRu/CeO₂—ZrO₂ (50 mol %) −25° C. 32 30 Pellet Ru/CeO₂—ZrO₂ (50 mol %)−30° C. 30 31 Pellet Co/CeO₂—ZrO₂ (50 mol %) −30° C. 25 32 HoneycombRu/CeO₂—ZrO₂ (50 mol %) −30° C. 33 Comparative Example 4 Pellet Ru (2 wt%)/CeO₂ −15° C. 0

A change with time of the temperature of the catalyst layer whenperforming the above test for a property of being activated at a lowtemperature is shown in FIG. 1 for each of the case where the catalyst(Ru/CeO₂—ZrO₂ (50 mol %)) of Example 17 was used and the case where thecatalyst (Ru (2 wt %)/CeO₂) of Comparative Example 4 was used. Further,a change with time of the concentration of each gas when the catalyst ofExample 17 was used is shown in FIG. 2.

As apparent from FIG. 1, in the case of Comparative Example 1 in whichthe catalyst support was CeO₂, a property of being activated was notexhibited when the initial temperature of the catalyst layer was −15° C.

On the other hand, in the case of Examples 15 to 32, by adding ZrO₂ toCeO₂ at 10 to 90 mol %, desirably at 20 to 70 mol %, it was found that aproperty of being activated was exhibited even at a temperature lowerthan room temperature.

In particular, regardless of the types of metals, i.e., noble metalssuch as Pt, Rh, and a Pt—Rh alloy, and base metals such as Co, Fe, Ni,Cu, Mo, and Mn, a property of being activated at a low temperature couldbe exhibited.

1. An ammonia oxidation/decomposition catalyst, comprising: a catalystsupport composed of a composite oxide of cerium oxide and zirconiumoxide; and at least one metal selected from the group consisting ofmetals of group 6A, group 7A, group 8, and group 1B as a catalyticallyactive metal deposited thereon, characterized in that the molarconcentration of zirconium oxide in the catalyst support is from 10 to90%.
 2. The ammonia oxidation/decomposition catalyst according to claim1, which is in a honeycomb form.
 3. The ammonia oxidation/decompositioncatalyst according to claim 1, which is in a pellet or raschig ringform.